1. Field of the Invention
The present invention disclosed in the present specification relates to a liquid crystal display using thin film transistors and, more particularly, to a liquid crystal display in which pixel switching circuits and peripheral driver circuits are integrally formed on the same substrate.
2. Description of the Prior Art
In recent years, techniques for fabricating semiconductor devices such as thin film transistors (TFTs) consisting of semiconductor thin films formed on cheap glass substrates have developed rapidly, because there is an increasing demand for active matrix type liquid crystal displays.
An active matrix type liquid crystal display has several tens to millions of pixel regions arranged in a matrix-like form and at least one TFT is disposed at each pixel. Electric charge going into and out of each pixel electrode is controlled by the switching function of each TFT.
The fundamental structure of the active matrix type liquid crystal display having TFT arrays is now described by referring to FIG. 1. Firstly, FIG. 1(A) is a cross-sectional view taken perpendicular to the substrates of the liquid crystal display. This cross section is equal to the cross sectional view along the broken line A-Axe2x80x2 of FIG. 1(B).
A substrate 101 is transparent to light, and an insulating film (not shown) is deposited on the surface of the substrate. Reference numerals show an active layer 102, a gate electrode 103, a data line 104, a drain electrode 105, an interlayer insulating film 106, a black matrix 107, pixel electrodes 108 comprising a transparent conducting film, and an orientation film 109.
The whole substrate having the constitution described above and having such TFT arrays thereon is hereinafter referred to as the active matrix substrate. In FIG. 1(A), only one pixel is shown, however, in practice, the active matrix substrate is constituted by pixel regions containing several tens to millions of pixel switching TFTs (referred to as pixel TFTs) and by peripheral driver circuit regions including a plurality of TFTs for driving the pixel TFTs.
On the other hand, reference numeral 110 shows a substrate having transparency, 111 shows a counter electrode consisting of a transparent conducting film, and 112 shows an orientation film. The whole of this substrate opposite to the active matrix substrate is referred to as a counter substrate.
As shown in FIG. 2(A), the active matrix substrate 203 and the counter substrate 204 are performed by orientation treatment to orient the molecules of the liquid crystal material in a given direction. Then, granular spacers 201 are uniformly dispersed on the active matrix substrate 203 to control the cell gap, or cell spacing, between the active matrix substrates 203 and the counter substrate 204. Thereafter, a sealing material 202 is printed. The sealing material 202 acts as an adhesive to mate the substrates each other and also as a sealant to prevent the liquid crystal material injected into the cell between the substrates from leaking from the substrate. Also, reference numeral 205 shows a driver circuit regions and 206 shows a pixel electrode. The direction of rubbing is indicated by 207.
FIG. 3 is a cross-sectional view of an active matrix substrate. As shown in FIG. 3, since granular spacers 301 are uniformly dispersed on the whole active matrix substrate 302 to control the cell gap, the spacers 301 exist in a peripheral driver circuit region 304, as well as in a pixel region 303. Usually, a pixel TFT 305 does not greatly differ in size from a driver circuit TFT 306. However, a black matrix covering the pixel TFTs 305 and pixel electrodes consisting a transparent conducting film are formed in the pixel region 303. Also, if the liquid crystal display is of the reflection type, reflective electrodes are formed in the pixel region 303. Furthermore, connecting wirings are formed in the driver circuit region 304 to constitute a CMOS circuit driving the pixel TFTs 305. Hence, the pixel region 303 differs in height above the surface of the substrate from the driver circuit region 304.
It is assumed that the pixel region is at a greater height above the substrate surface than the driver circuit region. The spacers are present in the driver circuit region, as well as in the pixel region. If granular spacers of almost uniform size are positioned in these two kinds of regions, the height above the substrate surface differs according to the spacer position. When, hp and hd are directed to the height of the top surface of each spacer located in the pixel region and in the driver circuit region, respectively, it can be seen that a difference in size between the pixel and driver circuit regions gives rise to a height difference h=hp ; hd.
Then, as shown in FIG. 4(A), the active matrix substrate 401 is mated with the counter substrate 402. Subsequently, a liquid crystal material 406 is injected between the active matrix substrate and the counter substrate, thereby, the entrance hole 403 is sealed with a sealant material (FIG. 4(B)). In this way, an active matrix type liquid crystal display having such a structure as shown in FIG. 1(A) is manufactured.
However, the liquid crystal display having such a structure as described above suffers from the following disadvantages.
Because of the height difference h due to the difference in size between the pixel region 404 and the driver circuit region 405, when the active matrix substrate 401 is mated with the counter substrate 402, it is impossible to make the cell gap uniform. This results in cell thickness nonuniformities. In addition, as shown in FIGS. 4(A) and 4(B), the counter substrate 402 is distorted. Because of these cell thickness nonuniformities and distortion of the counter substrate 402 in the liquid crystal display device, various defects are developed as follows; the image displayed on the liquid crystal display becomes nonuniform and also, interference patterns appear on the top surface of the counter substrate 402.
Where the driver circuit region is at a greater height than the pixel region above the substrate surface, extra force is applied to the spacers dispersed on the driver circuit region due to the height difference h when the active matrix substrate is mated with the counter substrate. Consequently, the driver circuit TFTs that are more complex in structure than the pixel TFTs are damaged not a less. As a result, the manufacturing yield is adversely affected.
Where granular spacers 115 are present on the pixel region as shown in FIG. 1(B), the orientation of the molecules of the liquid crystal material is disturbed near the spacers 115 and so disturbance of the displayed image (disclination) 116 may be observed.
Because of the height difference h described above, when the active matrix substrate is mated with the counter substrate, excess force is applied to the spacers dispersed on the driver circuit TFTs. Thus, the driver circuit TFTs that are more complex in structure than the pixel TFTs are damaged not a less. Consequently, the manufacturing yield is deleteriously affected.
Where the cell gap is controlled using the conventional granular spacers as mentioned above, a good display may not be provided due to various factors.
Also, where a liquid crystal display is manufactured as a commercial product or as a prototype, the cell gap would be generally set to about 5 to 6 xcexcm, regardless of the pixel pitch 117. In the future, liquid crystal panels with high definition will be required, and so the pixel pitch 117 will tend to be further fine.
For example, a projection type Liquid Crystal Display (projection) must have small panel size. Moreover, it must be designed to display an image with the high definition as possible because the image is projected onto a screen. Accordingly, in the future it will be necessary to manufacture LCDs with pixel pitches not higher than 40 xcexcm, preferably 30 xcexcm or less.
In this case, if the cell gap is kept at 5 to 6 xcexcm, it is considered that disclinations are produced possibly under the influence of a lateral electric field. Therefore, as the pixel pitch decreases, it is necessary to reduce the cell gap.
In the liquid crystal display that needs to produce an image with such high definition, even granular spacers of several micrometers lead to a deterioration of the display quality when they are present in the effective display region.
Also, liquid crystal displays using ferroelectric liquid crystals, which have attracted the attention recently, and reflection liquid crystal displays are required to have smaller cell gaps because of their characteristics.
However, it is generally difficult to manufacture a cell with a small and uniform cell gap by the use of conventional granular spacers.
In the conventional liquid crystal display in which pixel TFTs and driver circuit TFTs are integrally formed on the same substrate, a liquid crystal material and an insulating film exist also in the driver circuit regions because of the nature of the structure. The liquid crystal material and insulating film existing on the driver circuit regions produce unnecessary load capacitances on signal lines for driver circuit TFTs. In this case, these load capacitances attenuate signals flowing through the driver circuits. Hence, it is impossible to transmit signals faithfully to the pixel TFTs. Furthermore, load capacitances are produced between signal lines due to the liquid crystal material and insulating layer present between the signal lines adjacent or close to each other. As a result, a crosstalk is induced.
It is an object of the present invention to provide a semiconductor display device that is free from cell thickness nonuniformities and capable of displaying images with good quality by producing a cell having a small and uniform cell gap, which has been difficult by using conventional granular spacers. Also, in the case of using conventional granular spacers, it is another object of the invention to prevent unnecessary stress in the peripheral driver circuit TFTs when two substrates are mated together, and protect the driver circuit TFTs against damage. Further, it is an object of the invention to provide a semiconductor display in which load capacitances on driver circuits are minimized and which is capable of providing a good display without crosstalk.
A liquid crystal display according to the present invention at least comprising: a first substrate including a pixel region at least having a plurality of TFTs (thin film transistors) and a plurality of pixel electrodes electrically connected with the TFTs, and driver circuit regions which at least have driver circuits constituted by a plurality of TFTs for driving the a plurality of TFTs and which are provided in a place different from the pixel region; a second substrate opposite to the first substrate; a liquid crystal material held between the first and second substrates; a gap retaining material for controlling a gap between the first and second substrates; and a sealant; wherein the gap retaining material exists in regions outside of the pixel region and of the driver circuit regions. By this the above object is achieved.
The pixel region may be surrounded by the gap retaining material.
The driver circuit regions may be surrounded by the gap retaining material.
The gap retaining material may consist of any one of polyimide, acrylic, polyamide, and polyimidamide.
The gap retaining material may consist of a UV-curable resin or epoxy resin.
In a liquid crystal display according to the present invention at least comprising: a first substrate including a pixel region at least having a plurality of pixel electrodes and a plurality of TFTs electrically connected with the pixel electrodes, and driver circuit regions which at least have driver circuits constituted by a plurality of TFTs for driving the TFTs and which are provided in a place different from the pixel region; a second substrate opposite to said first substrate; a liquid crystal material held between the first and second substrates; a gap retaining material for controlling a gap between the first and second substrates; and a sealant; wherein the gap retaining material is a continuous wall surrounding the pixel region, and top surfaces of source lines for a plurality of TFTs constituting the driver circuit contact a substance having a smaller dielectric constant than top surfaces of the source lines of TFTs constituting the pixel region. By this the above object is achieved.
The pixel region may be surrounded by the gap retaining material.
The driver circuit regions may be surrounded by the gap retaining material.
Top surfaces of gate lines for a plurality of TFTs constituting the driver circuit may contact a substance having a smaller dielectric constant than top surfaces of the source lines and the gate lines of a plurality of TFTs constituting the pixel region.
The material having a smaller dielectric constant may be air.
The material having a smaller dielectric constant may be an inert gas.
The gap retaining material may consist of any one of polyimide, acrylic, polyamide, and polyimidamide.
The gap retaining material may consist of a UV-curable resin or epoxy resin.
The pixel electrodes and the TFTs electrically connected with the pixel electrodes may be arranged in a matrix-like form.
In a method of fabricating a liquid crystal display according to the present invention at least comprising the steps of: forming a first substrate including a pixel region at least having a plurality of TFTs and a plurality of pixel electrodes electrically connected with the TFTs, and driver circuit regions which at least have driver circuits constituted by a plurality of TFTs for driving the TFTs and which are provided in a place different from the pixel region; forming a second substrate opposite to the first substrate; forming a gap retaining material for controlling a gap between the first substrate and second substrates; exposing top surfaces of source lines of a plurality of TFTs constituting the driver circuits; bonding together the first and second substrates; and injecting a liquid crystal material between the first and second substrates; wherein the gap retaining material exists in regions outside of the pixel region and of the driver circuit regions. By this the above object is achieved.
This method of fabricating a liquid crystal display in accordance with the invention may further comprise the step of surrounding the driver circuit regions by the gap retaining material.
This method of fabricating a liquid crystal display in accordance with the invention may further comprise the step of exposing top surfaces of the gate lines of a plurality of TFTs constituting the driver circuits.
In a liquid crystal display according to the present invention at least comprising: a substrate including a pixel region at least having a plurality of TFTs and a plurality of pixel electrodes electrically connected with the TFTs, and driver circuit regions which at least have a plurality of driver circuits for driving the TFTs and which are provided in a place different from the pixel region; a gap retaining material formed on said substrate; and a liquid crystal layer in which a liquid crystal material is dispersed into a polymer material; wherein the gap retaining material exists in regions outside of the pixel region and of the driver circuit regions. By this the above object is achieved.
In the present invention, the cell gap is controlled by the gap retaining material extending continuously and surrounding the pixel region so that a small cell thickness that is uniform over whole of the transistor display device can be obtained.
Also, in the present invention, when the active matrix substrate is mated with the counter substrate, stress is produced neither in the pixel TFTs nor in the driver circuit TFTs. Consequently, neither the pixel TFTs nor the driver circuit TFTs are damaged.
Further, according to the invention, since no liquid crystal material is present in the driver circuit regions and, in addition, unnecessary insulating layer or the like have been removed, load capacitances on the driver circuits becomes small, so that the generation of crosstalk can be suppressed.
According to the present invention, since the cell gap is controlled by the continuously extending gap retaining material which surrounds the pixel region, it is possible to obtain a small and uniform cell thickness over whole of a semiconductor display device. Furthermore, according to the invention, when the active matrix substrate is mated with the counter substrate, stress is produced neither in the pixel TFT nor in the driver circuit TFT. Consequently, neither the pixel TFT nor the driver circuit TFT is damaged so that the production yield is enhanced. In addition, according to the invention, no liquid crystal material is present in the driver circuit region and, moreover, unnecessary insulating layer, etc. which would normally cover the driver circuit TFT is removed. This reduces the load capacitance on the driver circuit, and so the crosstalk can be suppressed.